> ## Documentation Index > Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt > Use this file to discover all available pages before exploring further. # C++ Maybe Unused Attribute The `[[maybe_unused]]` attribute is a standard C++17 feature used to suppress compiler diagnostic warnings generated when a declared entity is not referenced, evaluated, or instantiated within a translation unit. It explicitly signals to the compiler's diagnostic engine that the lack of usage is intentional, preventing the emission of `-Wunused` (or equivalent) warnings without altering the program's semantics, memory layout, or linkage. ## Syntax The attribute is applied using double square brackets and typically precedes the declaration of the entity, though placement can vary slightly depending on the grammar of the specific construct. ```cpp theme={"dark"} [[maybe_unused]] declaration; ``` ## Applicable Entities According to the C++ Standard, the `[[maybe_unused]]` attribute appertains to the following entities: * **Variables:** Local, global, and static variables. * **Functions:** Function declarations and definitions. * **Parameters:** Function parameters within a signature. * **Types:** Classes, structs, unions, and enumerations. * **Data Members:** Non-static data members of a class. * **Enumerators:** Individual values within an `enum` or `enum class`. * **Typedefs:** Type aliases and `typedef` declarations. ## Syntax Visualization The following code block demonstrates the syntactic placement of the `[[maybe_unused]]` attribute across various valid entities: ```cpp theme={"dark"} #include #include #include // 1. Applied to a function [[maybe_unused]] void calculateMetrics() { // ... } // 2. Applied to a function parameter void processBuffer(const char* buffer, [[maybe_unused]] std::size_t length) { // ... } // 3. Applied to a local variable void execute() { [[maybe_unused]] int executionStatus = 0; } // 4. Applied to a class/struct declaration class [[maybe_unused]] LegacyHandler { // ... }; // 5. Applied to a type alias (attribute must appear immediately after the identifier) using IdentifierMap [[maybe_unused]] = std::unordered_map; // 6. Applied to an enumeration and a specific enumerator enum class [[maybe_unused]] ConnectionState { Disconnected, Connecting, Connected, PendingRemoval [[maybe_unused]] }; // 7. Applied to a non-static data member struct Node { int id; [[maybe_unused]] void* internal_padding; }; ``` ## Compiler Behavior and Rules * **Diagnostic Suppression:** If an entity marked with `[[maybe_unused]]` is never referenced in the code, the compiler will suppress the diagnostic warning that would normally be emitted for that unused entity. * **Usage Permitted:** If an entity marked with `[[maybe_unused]]` *is* actually evaluated or referenced, the compiler ignores the attribute. No diagnostic warning is generated for using a "maybe unused" entity. * **Attribute Composition:** `[[maybe_unused]]` can be combined with other standard attributes within the same bracket pair using a comma-separated list. ```cpp theme={"dark"} [[maybe_unused, nodiscard]] int computeValue(); ``` * **Standardization:** Because it is a standard attribute (unlike compiler-specific extensions such as `__attribute__((unused))` in GCC/Clang or `#pragma warning(disable: 4100)` in MSVC), it is guaranteed to be parsed correctly by any C++17 compliant compiler.

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